A preferred application for the present invention is in tanks or vessels containing high voltage circuit breakers. Therefore, the background of the invention is described below in connection with such devices. However, it should be noted that, except where they are expressly so limited, the claims at the end of this specification are not intended to be limited to applications of the invention in a high voltage circuit breaker. For example, the invention disclosed herein may be employed in association with a circuit switcher, circuit breaker, load break switch, recloser, or the like.
A high voltage circuit breaker is a device used in the transmission and distribution of three phase electrical energy. When a sensor or protective relay detects a fault or other system disturbance on the protected circuit, the circuit breaker operates to physically separate current-carrying contacts in each of the three phases by opening the circuit to prevent the continued flow of current. In addition to its primary function of fault current interruption, a circuit breaker is capable of load current switching. A circuit switcher and a load break switch are other types of switching device. As used herein, the expression "switching device" encompasses circuit breakers, circuit switches, load break switches, reclosers, and any other type of electrical switch.
The major components of a circuit breaker or recloser include the interrupters, which function to open and close one or more sets of current carrying contacts housed therein; the operating mechanism, which provides the energy necessary to open or close the contacts; the arcing control mechanism and interrupting media, which interrupt current and create an open condition in the protected circuit; one or more tanks for housing the interrupters; and the bushings, which carry the high voltage electrical energy from the protected circuit into and out of the tank(s) (in a dead tank breaker). In addition, a mechanical linkage connects the interrupters and the operating mechanism.
Circuit breakers can differ in the overall configuration. However, the operation of most circuit breakers is substantially the same. For example, a circuit breaker may include a single tank assembly which houses all of the interrupters. U.S. Pat. No. 4,442,329, Apr. 10, 1984, "Dead Tank Housing for High Voltage Circuit Breaker Employing Puffer Interrupters," discloses an example of the single tank configuration. Alternatively, a separate tank for each interrupter may be provided in a multiple tank configuration. An example of a multiple tank circuit breaker is depicted in FIGS. 1A and 1B.
As shown in FIGS. 1A and 1B, the circuit breaker assembly 1 includes three cylindrical tanks 3. The three cylindrical tanks 3 form a common tank assembly 4 which is preferably filled with an inert, electrically insulating gas such as SF.sub.6. The tank assembly 4 is referred to as a "dead tank" because it is at ground potential. Each tank 3 houses an interrupter (not shown). The interrupters are provided with terminals which are connected to respective spaced bushing insulators. The bushing insulators are shown as bushing insulators 5a and 6a for the first phase; 5b and 6b for the second phase; and 5c and 6c for the third phase. Associated with each pole or phase is a current transformer 7. In high voltage circuit breakers, the pairs of bushings for each phase are often mounted so that their ends have a greater spacing than their bases to avoid breakdown between the exposed conductive ends of the bushings. Such spacing may not be required in lower voltage applications. The operating mechanism that provides the necessary operating forces for opening and closing the interrupter contacts is contained within an operating mechanism housing 9. The operating mechanism is mechanically coupled to each of the interrupters via a linkage 8.
During circuit breaker opening or closure, a high voltage potential develops across the contacts. As a result, an electrical arc can develop across the switch contacts, particularly the closer the contacts are to closure. It is desirable to minimize this arc. For this and other reasons, such circuit breakers are housed in tanks 3 which are then be filled with an inert gas such as SF.sub.6, which acts as an insulator to prevent arcing.
In order to ensure that the gas will perform its insulating task as design, it is important that the gas within the tank is maintained at about a preselected density. However, tanks may have leaks that over time allow the inert gas to escape from the tank. Hence, the density of the gas must be constantly monitored.
FIGS. 1A and 1B illustrate a prior art gas monitoring system. As illustrated in those Figures, a network of pipes 2 feeds the gas from each of the three tanks back to a single density monitoring device. As one might expect, if the density falls to an insufficient level, this design makes it difficult to determine the location, i.e., which tank is actually experiencing the leak and exposes all of the circuit breakers to failure from a leak in a single tank. Moreover, the intricate piping network also creates more places for leaks to occur.
The system of FIGS. 1A and 1B also includes a separate tank temperature monitor 15. The gas pressure and tank temperature are then fed into a control panel that calculates gas density.
Thus, there is a need for an improved gas density monitoring apparatus.